Update Window API for CSSOM smooth scrolling to match the spec

Source/core/frame/ScrollOptions.idl

Index: Source/core/frame/ScrollOptions.idl
diff --git a/Source/core/frame/ScrollOptions.idl b/Source/core/frame/ScrollOptions.idl
index 615da4b006a4fd237594236fd4dc0940c3389773..592ff49ba3e3f7ea4a4b845de38b5ef7437c36a8 100644
--- a/Source/core/frame/ScrollOptions.idl
+++ b/Source/core/frame/ScrollOptions.idl
@@ -2,8 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Spec: Non-standard
+// Spec: http://http://dev.w3.org/csswg/cssom-view#extensions-to-the-window-interface
+
+enum ScrollBehavior { "auto", "instant", "smooth" };
 
 dictionary ScrollOptions {
-    DOMString behavior;
+    ScrollBehavior behavior = "auto";
 };